Draft: Reduce redundant preloads in PipelineSerializer

What does this MR do and why?

Have a look at !136089 for an example of a very slow page (see flamegraph)

Various pages listing pipelines get extremely slow when the pipelines have a lot of builds, especially if they also have downstream pipelines with a lot of builds. This is wasteful because the preloaded builds are not actually used (a second request fetches the builds per stage instead), and we currently render at most 3 downstream pipelines per top-level pipeline.

By removing a few unused preloads, and reducing the number of downstream pipelines loaded on the page, we can drastically reduce the load time of these lists.

MR acceptance checklist

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Screenshots or screen recordings

Compare the wall flamegraph for pipelines.json before and after. Observe that the endpoint is much faster and does much less work:

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.